More than 80% of the male bass fish in Washington’s major river are now exhibiting female traits such as egg production because of a “toxic stew” of pollutants, scientists and campaigners reported yesterday.

Intersex fish probably result from drugs, such as the contraceptive pill, and other chemicals being flushed into the water and have been found right across the US.

The Potomac Conservancy, which focuses on Washington DC’s river, called for new research to determine what was causing male smallmouth bass to carry immature eggs in their testes. “We have not been able to identify one particular chemical or one particular source,” said Vicki Blazer, a fish biologist with the US geological survey. “We are still trying to get a handle on what chemicals are important.”

But she said early evidence pointed to a mix of chemicals – commonly used at home as well as those used in large-scale farming operations – causing the deformities. The suspect chemicals mimic natural hormones and disrupt the endocrine system, with young fish being particularly susceptible.